ESTRO 2021 Abstract Book

S1396

ESTRO 2021

Fig. 1 a) View of the phantom showing the layers and b) the Phantom placed on a track attached to the motion motorpositioned inside the MR.

Image quality in terms of SNR and uniformity was assessed using the ACR phantom. The assessment of geometric distortion and image quality was performed for typical pulse sequences used in RT planning (2D T1, 2D T2 and 3D cube). An in-house software module which automatically locates features and performs an automatic image quality control was developed. Results The table below shows the geometric distortion for all the sequences. Up to a radial distance of 150 mm, the mean distortion was 0.7 mm for the Axial T1, 0.6 mm for the axial Cube T2, 0.9 mm for the sagittal T1 and 0.8 mm for the sagittal T2.

Table1: Geometric distortion for the different sequences. Sequence Radial distance

Mean Distortion

Axial T1

50 mm

0.3 mm

100 mm

0.4 mm

150 mm

0.7 mm

Axial Cube T2

50 mm

0.2 mm

100 mm

0.3 mm

150 mm

0.6 mm

Sagittal T1

50 mm

0.4

100 mm

0.5 mm

150 mm

0.9 mm

Sagittal T2

50 mm

0.3 mm

100 mm

0.4 mm

150 mm

0.8 mm

Table 2 shows the image quality assessment results. The highest SNR was recorded for the axial cube T2sequence while the lowest SNR was recorded for thesagittal T2sequence. Nosignificant difference inuniformity has been recorded between the sequences.

Table2: SNR and uniformity measuredfor different sequences. inside the MR. Sequence SNR Uniformity Axial T1 137 34.3

Axial Cube T2

159

33

Sagittal T1

91

37

Sagittal T2

55

38

Conclusion This study aimed at investigating the 3D geometric distortion and the quality of MR images used for spinal SBRT. Results showed that the geometric distortion are within the tolerances. Further investigation on the dosimetric impact of the MR distortion will be performed for various DVH parameters.